Radio-loud Narrow Line Seyfert 1 under a different perspective: a revised black hole mass estimate from optical spectropolarimetry

Baldi, Ranieri Diego, Capetti, Alessandro, Robinson, Andrew, Laor, Ari and Behar, Ehud (2016) Radio-loud Narrow Line Seyfert 1 under a different perspective: a revised black hole mass estimate from optical spectropolarimetry. Monthly Notices of the Royal Astronomical Society: Letters, 458 (1), L69-L71. (doi:10.1093/mnrasl/slw019).

Abstract

Several studies indicate that radio-loud (RL) active galactic nuclei (AGNs) are produced only by the most massive black holes (BH), MBH ∼ 108–1010 M⊙. This idea has been challenged by the discovery of RL Narrow Line Seyfert 1 (RL NLSy1), having estimated masses of MBH ∼ 106–107 M⊙. However, these low MBH estimates might be due to projection effects. Spectropolarimetry allows us to test this possibility by looking at RL NLSy1s under a different perspective, i.e. from the viewing angle of the scattering material. We here report the results of a pilot study of Very Large Telescope spectropolarimetric observations of the RL NLSy1 PKS 2004−447. Its polarization properties are remarkably well reproduced by models in which the scattering occurs in an equatorial structure surrounding its broad-line region, seen close to face-on. In particular, we detect a polarized Hα line with a width of ∼9000 km s−1, ∼6 times broader than the width seen in direct light. This corresponds to a revised estimate of MBH ∼ 6 × 108 M⊙, well within the typical range of RL AGN. The double-peaked polarized broad Hα profile of the target suggests that the rare combination of the orientation effects and a broad line region dominated by the rotation might account for this class of objects, casting doubts on the virial estimates of BH mass for type-I AGN.

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